Hi,
In order to make the complete chain 50 ohms you also need to exchange the calibration part. I did the conversion some time ago and it is relative easy. The official guide was posted in a key sight blog. It looks complicated but is actually adjusting some resistors and BNC chassis parts(be careful, the center pin on 75 ohm version is different from 50 ohm)
Found here :
https://community.keysight.com/thread/5431----------
When using an external matching pad or network, the analyzer does not know that it is connected, so you have to compensate for that 5-6 dB front end loss for sure. If you plan to convert the analyzer, you can find the details below. Since your analyzer is very old, you may not be able to get the conversion parts needed, not sure.
Agilent 8590B and HP 8591A Conversion for 50 or 75 ohm Operation
________________________________________
Parts listed for the impedance input required. Order one of each.
75 Ohm Description 50 Ohm
0590-1251 NUT HEX 15/32-32 NONE
08590-20182 CBL AY-RF INPUT 08590-20157
08590-60179 RF INPUT 1250-2191
NONE ADPT M N - F BNC 1250-0780
6960-0150 PLUG-HOLE 6960-0148
08590-60028 Cable, Cal Out 8120-5052
5062-6452 (75ohm) CABLE, CAL Out-RF In 8120-2682 (50ohm)
0955-0453 A24A4 ATTENUATOR, TG OUTPUT 0955-0453
08590-20179 J2 MATCHING PAD NONE
Agilent 8590B Dress Panel
75 Ohm Description 50 Ohm
08590-00039 HARD KEYPAD 08590-00038
08590-00046 RUBBER KEYPAD 08590-00045
HP 8591A Dress Panel
75 Ohm Description 50 Ohm
08591-00002 HARD KEYPAD 08591-00001
08591-00014 RUBBER KEYPAD 08591-00013
Common to the HP8590B and the Agilent 8591A conversions.
To convert a 50 ohm 3RD CONVERTER, p/n 08590-60192, 08593-60021 or 08591-60081 to 75 ohm, order the parts listed in the 75 ohm column and replace the parts listed in the 50 ohm column.
To convert a 75 ohm 3RD CONVERTER, p/n 08590-60193 or 08590-60344 to 50 ohm, order the parts listed in the 50 ohm column and replace the parts listed in the 75 ohm column.
75 Ohm Description 50 Ohm
0698-7239, 1.33 KOHM A9R20 0698-7237, 1.1 KOHM
0698-7216, 147 OHM A9R21 0698-7212, 100 OHM
0698-7216, 147 OHM A9R23 0698-7212, 100 OHM
9135-0076, 39 NH A9L2 9135-0070, 24 NH
The RF input cable (W10) that connects to the rear of the RF Input connector remains the same for both configurations since it attaches to the rear of the Minimum Loss Adapter or the N connector.
Refer to "STOR PWR ON UNITS" in Chapter 13 (Service Softkey Descriptions) in the service manual for setting the Impedance and Power-On units of the analyzer to 50 or 75 ohm input.
Special considerations for 75 ohm spectrum analyzers.
If the final configuration is 75 ohms (Opt. 001), use only the 75 ohm cable (5062-6452) for calibration and use only 75 ohm cables for signal input. Using a 50 ohm cable can damage the 75 ohm input connector which is the Minimum Loss Adapter (MLA). The replacement cost of the MLA is currently $220. The MLA is not field repairable.
The recommended 75 ohm F to 75 ohm BNC connector p/n is 1250-0076, using Radio Shack type connectors WILL eventually damage the input connector because it is a 50 ohm BNC connector.
If converting an Option 011 (75 ohm TG) to 50 ohms, remove the MLA (TG Output Adapter, 50 to 75 ohm) and re- place with the type N (f), p/n 1250-2191, connector.
Special Considerations for the Agilent 8590B with a Count Lock, Option 013
If the 8590B has a Count Lock Option (Option 013), the output attenuator (0955-0453) will have to be added to the TG output or the TG Cal will fail. The reason is the analyzer will now ID as an Agilent 8591A, so when the Cal TG test routine is run the firmware will attempt to change the TG attenuator and test the results.
Regards,
Eric